The present invention relates to a method for adhesive bonding of an overlay film to an acrylic resin board. More particularly, the invention relates to an improved method for adhesive bonding of a decorative overlay film of a plastic resin to an acrylic resin board by using a hot-melt adhesive to give a decorative acrylic resin-based overlaid board having improved stability of overlaying lamination in a prolonged service under adverse ambient conditions or in mechanical working.
As is well known, acrylic resins have various excellent properties such as high clarity, mechanical strengths, dimensional stability in molding and the like so that they are used in a wide field of applications not only as a material of parts of various instruments and machines but also as a material of various kinds of indoor and outdoor utilities such as, for example, doors, partition boards, arcade roofs, signboards, doorplates, telephone booths and so on, in most cases, in the form of a board having a thickness in a wide range from less than 1 mm to several centimeters.
When decorativeness or a shielding effect to light or heat radiation is desired in these acrylic resin boards not obtained with a plain resin board, it is a conventional technology that the acrylic resin board is overlaid with an overlay film of a plastic resin provided with a designed pattern by printing, with a metallic layer over whole surface or in a pattern by sputtering, vapor deposition or other suitable methods and with an embossment. The acrylic resin board and the overlay film are bonded together usually by using a hot-melt adhesive in consideration of the workability of the laminated resin board in the subsequent works such as thermal shaping or mechanical working, e.g., bending, cutting, sawing, drilling, shaving and the like.
A problem in such an overlaid acrylic resin board is that the adhesive bonding between the resin board and the overlay film by use of a hot-melt adhesive is not always very reliable in respect of stability of bonding in the subsequent works of the overlaid board or during a prolonged service of the utilities made from the overlaid acrylic resin board, especially, under very adverse ambient conditions. For example, delamination of the layers may eventually be caused in the mechanical working such as sawing and shaving for trimming of the periphery. Further, blistering of the overlay film is sometimes found in the overlaid acrylic resin board when it is prolongedly exposed to direct sunlight as is unavoidable in outdoor utilities. The requirements for overlaid acrylic resin boards include, for example, resistance against crazing, resistance against organic solvents and plasticizers, stability against blistering, adaptability to thermal forming, bendability to comply with a curved surface, adaptability to job-site mechanical working such as drilling, and so on. Various proposals and attempts, of course, have been made to solve the above mentioned problems relative to the improvement in the stability of overlaying lamination and to satisfy the various requirements mentioned above in overlaid acrylic resin boards but none of the prior art methods is quite satisfactory having its own merits and demerits.